It is known that smart drive for activating a device is performed in a so-called smart system. Smart drive is performed, for instance, to unlock doors or start a vehicle drive device by controlling a portable device to receive a request signal transmitted from a vehicle and to respond an reply signal from the portable device to the vehicle.
It is anticipated that the smart system may suffer a relay attack. Even when the portable device is positioned at a communication incapable distance from the vehicle, a malicious person may perform a relay attack by indirectly establishing communication between the portable device and the vehicle using repeaters and cause the vehicle to perform smart drive.
In a relay attack, a repeater A of a first malicious person is positioned within a communication range of the vehicle, and a repeater B of a second malicious person is positioned near the portable device within a communication range of the vehicle mounted device. The repeater B receives a request signal transmitted from the vehicle, converts the request signal to a relay signal having a long-distance communication range, and transmits the relay signal to the repeater B. Further, the repeater B receives the relay signal, converts the received relay signal to the request signal, and transmits the request signal to the portable device. When receiving the converted request signal from the repeater B, the portable device transmits reply signal. When receiving the reply signal from the portable device, the vehicle mounted device controls the vehicle to perform smart drive.
As a countermeasure technology for the above-described relay attack, a technology disclosed in Patent Literature 1 makes use of a delay in a response that is caused when a repeater operates during the time interval between the instant at which the vehicle transmits the request signal and the instant at which the vehicle receives the reply signal. The technology disclosed in Patent Literature 1 recognizes the received reply signal as an authenticated reply signal only when the time interval between request signal transmission time and reply signal reception time is equal to or shorter than a predetermined effective duration.
Meanwhile, as a countermeasure technology for the above-described relay attack, another technology disclosed in Patent Literature 2 performs common logical operations based on collation data to randomly change frequencies so that the same reception frequencies and transmission frequencies are used at the vehicle and the portable device.
In the technology disclosed in Patent Literature 1, in order to accurately recognize an illicit reply signal involved in a relay attack, it is necessary that the effective duration needs to be set to a relatively short duration. However, a problem is that responsiveness also decreases with a decrease in the effective duration. More specifically, even when an authenticated reply signal is received with a delay due to a delayed response caused by temperature or aging of the devices, the authenticated reply signal may be erroneously recognized as an illicit reply signal.
In the technology disclosed in Patent Literature 2, even when the frequency of the request signal is changed, in a case where a repeater relays the request signal with only amplitude amplification, the portable device may inadvertently transmit a reply signal in response to the request signal.